Since an automobile during engine operation produces exhaust gases at high temperatures of 900° C. or more, exhaust system parts, such as an exhaust manifold, a catalyst system, pipes, a muffler, and the like undergo high temperatures. Therefore, a large number of heat shielding covers are provided in their peripheries for the purposes of prevention of thermal damage and prevention of burn injury. Further, since these heat shielding covers are in many cases provided in narrow and extensive ranges in the vicinities of the high-temperature exhaust system parts, they often tend to be such complex and large-sized covers as to conform to the shape of matching members. In addition, control of CO2, which has its inception in the issue of global warming in recent years, is an important challenge, and, for automobiles, individual parts are required to be more lightweight. In particular, in the case of the heat shielding covers around automobile exhaust systems whose temperatures tend to be high, as described above, the number of places where they are used is ever increasing, and the light weight has been an extremely important task.
Conventionally, deep-drawn products of steel sheets (galvanized steel sheets, aluminized steel sheets, or the like are actually used due to problems in rust prevention) have been frequently used as these heat shielding covers. Since the steel sheets exhibit elongation required for deep drawing and have sufficient strength and rigidity, the steel sheets have satisfied shape retainability and durability against stone bounding and the like which are required as the heat shielding covers. However, large-sized heat shielding covers weigh as much as several tens of kilograms, and fixing portions for supporting them are also required to have strength and durability, so that tendencies toward greater size and heavier weight are naturally underway, which is contrary to the tendency toward lighter weight. In addition, even in sheets which excel in elongation such as steel sheets, low-length portions are partially present in deep drawing which is accompanied by reduced sheet thickness, and stress-concentrated portions where fracture can occur during deep drawing forming or which can be a cause of fracture are inherent. Hence, there are frequent defects in which these stress-concentrated portions lead to breakage in high-load environments (high temperature, high vibration, salt damage environment, long-time assurance, etc.) as in automobiles. The prevention of starters of these fractures is also an important task in providing a highly reliable heat shielding cover.
In order to overcome these problems, a number of inventions have already been made and put to practical use. For example, a heat shielding cover is known in which semispherical protrusions (embosses) having complex shapes and different diameters are imparted to a steel sheet or an aluminum sheet by drawing (refer to patent document 1). According to this publication, it is possible to provide high rigidity for a sheet of equal thickness by the portion of the protrusion, and since the shape retainability increases, the function of the heat shielding cover can be demonstrated, and light weight can also be realized. However, since the imparting of the protrusions is dependent upon drawing, the molding of the cover shape is dependent upon the material characteristics which the original sheet has. As long as a steel sheet having an elongation rate of several tens of percent is used, the case would be different, but in the case of an aluminum sheet several percent to ten-odd percent is a limit of its elongation rate, and it is difficult to say that sufficient deep drawability can be ensured. Furthermore, as for the protruding portions, the sheet thickness becomes thinner than the planar portions, so that the strength is low, and there are cases where cracks, pinholes, or the like are produced during the shape forming.
In addition, a sheet is also known in which ridges having inwardly curved side walls are regularly arranged in a two-dimensional plane by bending (refer to patent document 2). According to this publication, the shape retainability as a cover is improved by the rigidity which the ridges possess. In addition, as the material stored in the ridges having reentrant side walls returns to its original shape owing to the molding force during the molding of the cover, moldability similar to that of deep drawing is consequently demonstrated, and the percentage of the material stored in the ridges becomes equivalent to the elongation rate in principle. For this reason, moldability into a lightweight and complex shape is provided without producing a reduction in the sheet thickness within that range, and it becomes possible to ensure durability in a high-load environment. However, if the sheet is compressed in its thickness direction during the press molding of the cover, the material stored in the ridge portions is released and at the same time undergoes shrinkage in ridges in their vicinities, but an inverse folding force is loaded to the reentrant portions in the ridges. Since a metal sheet is ordinarily work hardened during machining such as side wall forming by strike bending, and the aluminum sheet or the like, in particular, has a small elongation rate, there is a possibility that the bent portion becomes fractured due to the inverse bending force loaded on the reentrant side wall, and it is apprehended that this fractured portion may become a flaw in a vibrational environment, and a very small fracture may progress and lead to the breakage of the cover. In addition, there are cases where a crack occurs along the ridge during molding.
Patent Document 1 JP-A-2000-136720
Patent Document 2 JP-T-2001-507282